Bacteriostatic water vs sterile water
Both are sterile water. The difference is a preservative: bacteriostatic water contains benzyl alcohol (typically 0.9%), which limits bacterial growth so the vial can be entered more than once.
| Bacteriostatic water | Sterile water | |
|---|---|---|
| Contents | Sterile water + benzyl alcohol (0.9%, some products 1.1%) [1] | Sterile water only |
| Container | Multiple-dose: repeated withdrawals allowed [1] | Typically single-dose |
| After first entry | Up to 28 days under USP <797>, unless the manufacturer specifies otherwise [2] | Single use; discard the remainder |
| Storage | 20–25 °C per the Pfizer label [1] | Per its label |
| Compatibility | Benzyl alcohol is incompatible with some compounds [1] | No preservative to interact |
Why the benzyl alcohol matters
The preservative is what allows a vial to be punctured more than once. Without it, an opened vial of sterile water has no protection against contamination and should be treated as single-use.
Reading "28 days" correctly
The 28-day figure is the USP <797> beyond-use date for multiple-dose containers once first entered, applying "unless otherwise specified by the manufacturer" [2]. It refers to the diluent vial, not to a peptide solution made with it; peptide solutions have their own, usually shorter, stability (see how to store peptides).
When sterile water, or another solvent, is the better choice
Use preservative-free sterile water when benzyl alcohol isn't compatible with the compound or the experiment [1]. Some peptides don't dissolve well in water at all and call for dilute acid, dilute base or a co-solvent; check the compound's documentation (see how to reconstitute peptides).
Bac water from XS
XS carries bacteriostatic water in 10 mL vials for $14. It ships with any order and is same-day in Las Vegas and Atlanta.
Questions
What is the difference between bacteriostatic water and sterile water?
Bacteriostatic water contains benzyl alcohol (typically 0.9%) as a preservative and comes in multiple-dose vials. Sterile water has no preservative and is single-use once opened.
How long is bacteriostatic water good for after opening?
Under USP <797>, a multiple-dose container has a beyond-use date of 28 days after first entry unless the manufacturer specifies otherwise.
Can I use sterile water instead of bacteriostatic water?
Often, yes. Sterile water works as a diluent and avoids benzyl alcohol, but it has no preservative, so treat the opened vial and solutions made with it as single-use.
References
- Pfizer: Bacteriostatic Water for Injection, prescribing label
- Minnesota Dept. of Health: Safe injection practices (USP <797> beyond-use dates)
References describe general laboratory practice from the cited sources; stability and handling vary by compound. Follow the documentation for the specific peptide.